1. Field of the Invention
The present invention relates to an organic thin-film solar cell element wherein plural layers each comprising an electroconductive polymer material are laminated.
2. Description of the Related Art
In order for organic thin-film solar cells to exhibit high photoelectric conversion performance, it has been desired to develop a photoelectric conversion layer capable of sufficiently absorbing solar light energy and efficiently converting the absorbed solar energy into electric energy.
Many organic semiconductor materials used in photoelectric conversion layers are insufficient in optical properties essential for the photoelectric conversion layers, for example, as follows: the absorption wavelength range thereof is narrow; and the extinction coefficient is small. For this reason, only rays having a specific wavelength range of solar rays (white rays) can be used for power generation. This is one of the reasons why the photoelectric conversion efficiency is low. Thus, according to a photoelectric conversion layer wherein plural layers are laminated, organic semiconductor materials having different absorption wavelength ranges are used in the respective layers, whereby light can be absorbed within a broad wavelength range. Additionally, the lamination of the plural layers makes the thickness of the photoelectric conversion layer large. Thus, it is expected that the absorbance can be made large.
The photoelectric conversion performance of organic thin-film cellar cells is lower than that of crystalline solar cells. One of the causes therefor is that the resistance barrier in the interface between their photoelectric conversion layer, which has a function of converting light energy into electric energy, and their electrode layer is large. If the resistance barrier in this interface is large, electric charges generated inside the photoelectric conversion layer cannot be taken out into an external circuit with a high efficiency.
In order to solve this problem, it is effective to form a charge taking-out layer between the photoelectric conversion layer and the electrode layer. This causes the resistance barrier between the photoelectric conversion layer and the electrode layer to be reduced, so that the transfer of electric charges is promoted.
In general, an organic semiconductor material is used in a photoelectric conversion layer and a charge taking-out layer. Organic semiconductor materials can be roughly classified into high-molecular materials and low molecular materials. The high molecular organic semiconductor materials have advantages that they can each be formed into a film by the coating thereof and further the formation of the thin film is easy. In particular, electroconductive polymer materials are excellent in film-formability.
However, when plural layers are formed into a lamination form by the coating, there is caused a problem that constituents of a lower layer out of the layers elute into a solvent in the coating solution for forming an upper layer, out of the layers, which is to be arranged on the lower layer. In general, therefore, as the solvent for the upper layer forming coating solution, there is used a solvent wherein the constituents of the lower layer are not dissolved at all (see, for example, C. W. Tang, “Two-layer organic photovoltaic cell”, Applied Physics Letters, vol. 48, No. 2, pp. 183-185 (1986)). However, many of the polymeric organic semiconductor materials, in particular, almost all of electroconductive polymeric materials are dissolved in organic solvents having similar natures. It is therefore difficult to use difference in solubility in a solvent to laminate plural layers.
Conventionally, there has been known a method of using the above-mentioned method, wherein difference in solubility in a solvent is used, to laminate a hole transporting layer comprising a high molecular p type organic semiconductor material and an electron transporting layer comprising a high molecular n type organic semiconductor material by coating, so as to form a photoelectric conversion layer. In this process, difference between dielectric constants of solvents is chiefly used. However, according to this process, the high molecular organic semiconductor materials and solvents, which can be used, are restricted. Even if two layers can be made by laminating onto another layer by coating, it is very difficult to dissolve a high molecular organic semiconductor material into a solvent having a low dielectric constant suitable for a third layer since the solvent used to form the third layer is required to have a lower electric constant than the solvent used to form the second layer. For this reason, the upper limit according to this process is that two layers of a hole transporting layer and an electron transporting layer are laminated onto each other. Thus, in reality, three or more layers cannot be laminated onto each other.
In the case of forming an electron hole transporting layer comprising both of a high molecular p type organic semiconductor material and a high molecular n type organic semiconductor material, high molecular organic semiconductor materials and solvents which can be used are further limited. Thus, even if difference between dielectric constants of solvents is used, it is difficult to laminate plural electron hole transporting layers onto each other by coating.
Accordingly, there has not been any report about an organic thin-film solar cell element wherein plural photoelectric conversion layers are directly laminated onto each other.
Conventionally, the following process has been known; a process of using an aqueous mixture dispersion of polyethylenedioxythiophene/polystyrenesulfonic acid (PEDOT/PSS) to laminate plural layers. This is also the above-mentioned process, wherein difference in solubility into a solvent is used, and is a process of coating an aqueous mixture dispersion of PEDOT/PSS to form a first layer, and next coating, thereon, a solution wherein a high molecular organic semiconductor material is dispersed into an organic solvent to form a second layer. This PEDOT/PSS does not have a photoelectric conversion function, but has a function of stabilizing the taking-out of holes. Thus, the formation of the layer made of PEDOT/PSS makes it possible to improve the hole taking-out efficiency.
However, PEDOT/PSS is a special material out of high molecular organic semiconductor materials, and can be dispersed in water. On the other hand, among high molecular organic semiconductor materials other than PEDOT/PSS, materials, which can be dispersed in water, are almost unknown. For this reason, there is not any report about an organic thin-film solar cell element wherein a photoelectric conversion layer and a charge taking-out layer wherein a high molecular organic semiconductor material other than PEDOT/PSS is used are directly laminated onto each other.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2003-264085 discloses an organic film solar cell and an organic electroluminescent element wherein plural functional organic thin-film layers are laminated onto each other through an electroconductive thin-film layer. According to the JP-A No. 2003-264085, it is disclosed that a high molecular organic semiconductor material can be used in this functional organic thin-film layer or electro conductive thin-film layer. However, about the lamination of plural layer by use of a high molecular organic semiconductor material, only an example wherein the aqueous dispersion of PEDOT/PSS is used is given in working examples therein.